It Ain’t Easy Bein’ Green
Levasseur
Virtual Lab Dissection: http://mhhe.com/biosci/genbio/virtual_labs/BL_16/BL_16.html
The Northern Leopard Frog (Rana pipens) is a green grass frog found in marshes and ponds, but will often
venture into well-covered grasslands as well, earning them their other common name, the meadow frog.
They are most commonly found in the northern parts of the United States and in Canada.
Northern leopard frogs are so named for the array of irregularly shaped dark spots that adorn their backs and
legs. They are greenish-brown in color with a pearly white underside and light-colored ridges on either side of
their backs. They are considered medium-size, reaching lengths of 3 to 5 inches nose to rump. Females are
slightly larger than males. During the breeding season, males have swollen thumbs to aid in mating. Breeding
occurs in ponds, swamps, and other stationary water sources. Eggs are attached to vegetation or may lie idle
at the bottom of a pond and may be deposited in masses of 300 to 800 eggs. Hatching occurs in about a week
and tadpoles develop for one year, transforming upon reaching a size of 2 to 3 inches.
They are generally sluggish animals that stay in one place a long time waiting for food to hop or fly by. The
croak of the male is to call a female or to establish territory. As tadpoles, Leopard Frogs feed on algae and
other plants. As mature frogs, they will eat just about anything they can fit in their mouths. They sit still and
wait for prey to happen by, then pounce with their powerful legs. They eat beetles, ants, flies, worms, smaller
frogs, (including their own species), and even birds and garter snakes.
Fast Facts: Type: Amphibian; Diet: Carnivore; Average life span in the wild: 2 to 4 years; Size: 3 to 5 inches;
Group name: Army; Interesting Fact: A genetic mutation gives rise to the Burnsi leopard frogs, which have no
spots.
Purpose: To become familiar with amphibian anatomy and comparative anatomy to humans.
Materials: preserved frog; dissecting pan; dissection kit (scalpel, scissors, probes, forceps); dissection pins
External Structures
Procedure: Part 1—Dorsal (back) Structures
A. Rinse your frog in the sink to remove any preservative.
B. Spread the frog, belly down, on the dissecting pan. Locate the following structures: eyes, head,
nostrils, tympanum (ear covering), thumb, forearm, hind leg, webbed hind foot and label them on
diagram 1 of your data sheet.
Your labeled diagrams will serve as your data.
Your question answers will serve as your analysis for this lab. Complete sentences are required.
Questions: Answer the questions under the “data” section of your lab write-up. Answer in complete
thoughts where necessary.
1. What adaptive function does the tympanum serve (remember, the frog lives in the water)? Why is it
positioned where it is, in relation to its eyes?
2. Observe the thumb of your frog. Is it large or small? Large thumbs indicate a male frog. What do you
think the sex of your frog is? Why would this be of adaptive value (think about reproduction)? Briefly
explain how frogs mate.
3. What characteristics of the frog’s limbs suggest that it lives in the water?
Procedure: Part 2—Ventral (stomach) Structures
C. Turn the frog over to observe the ventral, or bottom side. On diagram 2 of your data sheet label
the head, thorax, and abdomen.
Questions:
4. What adaptive value is the contrast in color from the frog’s dorsal side to its ventral side (think about
living in the water and who its predators might be)? Be specific!
Procedure: Part 3—the Mouth
D. Keeping the frog on its back, pry open the mouth.
E. With care, with scissors, cut the mandible at the corners of the mouth on either side so that it will
remain open. You may pin down the bottom jaw with dissecting pins to hold it open.
F. Locate the frog’s tongue. It should be one of the first structures you see as you look into the
mouth. Label the tongue on diagram 3.
G. Using diagram 3, find and label the openings to the Eustachian tubes (ears), located on either side
of the mouth, toward the back.
H. Find and label the vomerine teeth, on the upper jaw toward the middle front of the mouth, and
the maxillary teeth which make a semi-circle around the upper jaw.
I. Find the flat region in the middle of the upper jaw. Label this the palate. Find and label the glottis
in the back of the throat.
J. Label the nostrils. You can find them by following the nose holes from the outside.
K. Finally, label the vocal sac openings and the point of attachment of the tongue.
Questions:
5. What is unusual about the place of attachment and position of the frog’s tongue? Why do you suppose
this is? (think of how a frog catches its prey)
6. How are the vomerine and maxillary teeth different? What do you think is the purpose of this
difference (think about what frogs eat)?
Internal Structures
Procedure: Part 4—Muscles
j. With your frog on its back (belly up), use a scalpel or scissors to CAREFULLY cut through the frog’s
skin. Make very shallow cuts to prevent damaging other tissue.
k. Cut the skin all the way around the neck.
l. Use forceps and fingers to completely peel away the cut skin. Use the scalpel to cut attached skin as
you pull. All frog skin needs to be thrown away in the trash.
m. Continue until you have completely skinned the frog.
n. Muscles work in pairs. One contracts (shortens) while the other relaxes (lengthens). Bend one of the
back legs at the knee. Notice the muscle movement. Draw two muscles on the leg of your diagram 2
and label one “contract” and one “relax” for when you are pulling the leg out.
Questions:
7. Compare the size of the muscles in the front legs to those in rear. Which are larger? Why do you
suppose that is?
Procedure: Part 5—Respiratory and Circulatory Systems
o. Using a scalpel, cut your frog along the dotted lines as in diagram 2. Cut CAREFULLY, but try to cut all
the way through muscle wall. Pull back the muscle flaps and cut them off (the large reddish-gray organ
you see is the liver, do not cut it out). You do not want to damage any internal organs. To view the
heart and lungs, you need to cut out the pectoral girdle. Grasp the sternum with forceps and cut the
clavicle away from the scapula and humerus. Remove the pectoral girdle so the heart and lungs can be
observed.
p. If the body is full of a black mass (this is not very likely), cut it out. These are eggs and need to be
removed to see the other organs.
q. Locate and label right and left lungs in the upper portion of the chest cavity on diagram 4. They
appear as small peanut shaped organs underneath the other organs, one on each side of the cavity.
Find and label the right and left bronchiole attached to each lung. Trace the bronchi to the larynx
(voicebox) and label it.
r. Remove the heart by first carefully cutting away the thin tissue covering the heart (pericardium).
s. Cut the blood vessels connecting the heart and remove the heart. Label the heart and aortic arch on
the diagram.
Questions:
8. Frogs have simple single-chambered lungs while mammals and birds have millions of tiny air sacs in
their lungs (called alveoli). Why might the frog need to obtain air through its lungs as well as through
its skin? How will this limit the places a frog can live?
9. What might be the purpose of the larynx? How would this help in the mating process?
10. The frog has a three chambered heart. Why do you suppose this is less efficient than the 4-chambed
mammalian heart?
Procedure: Part 6—Digestive System
t. After removing the heart and lungs, the large greenish, three sectioned organ is the liver. Its
function is to produce bile to help digest fats. Lift the liver up and look underneath, you should see a
small green sac attached to the liver called the gall bladder. Its function is to store bile for use by the
stomach. Label the liver and gall bladder on diagram 5.
u. The large tube like organ to the right is the stomach. The stomach breaks down proteins with the
help of hydrochloric acid. A small finger-like attachment under the stomach is the pancreas. Its job is
to produce chemicals that aid in digestion and produce insulin. Label these organs.
v. Follow the stomach until it gets narrower and begins to twist and turn. These are the small
intestines. Their job is to absorb nutrients into the blood. As the small intestines get larger near the
back end of the frog, they become the large intestines (they absorb water into the blood). The opening
for waste out of the body is called the cloaca. Label these structures and list the function next to the
label. Also label the human digestive system (diagram 7) using your book if necessary. (stomach,
small intestine, large intestine, gall bladder, appendix, liver, pancreas, rectum, salivary glands,
esophagus, mouth, and anus)
Questions:
11. Notice all the blood vessels attached to the digestive organs. Explain the connection between the
digestive and circulatory systems.
Procedure: Part 7—Urogenital System
w. Carefully remove the stomach and intestines. You may see long, yellow, finger-like structures called
fat bodies. These are surrounding the reproductive organs. Fat bodies are unique to animals that
cannot control their body temperature. Beneath these bodies you can see either the ovaries (female)
or the testes (male). Long curly tubes will accompany the structures if they are females. These are the
oviducts which carry the eggs. If your frog does not have these tubes, but has two red bean-like organs
(testes) then you have a male. Return to question 2 and change the sex of your frog if necessary.
Questions:
12. Given the fact that frogs hibernate in the mud all winter and can’t control their body temperature,
what do you suppose the function is for the fat bodies?
13. Where are the testes in the frog (internal or external)? How is this different from humans? What is the
purpose of this difference?
14. The female frog does not have a uterus. Why not?
Procedure: Part 8—Skeletal System
x. Cut the muscles away from one upper leg and one lower leg.
Questions:
15. What do you notice about the foreleg bones compared to those of a human? Why do you suppose this
is? How does this show an evolutionary adaptation?
Conclusion:
After completing the lab dissection, in a well-organized conclusion, discuss the differences in
amphibian anatomy and physiology (how things work) from what you have learned about human
anatomy and physiology. Discuss both external and internal anatomy. Don’t forget to include what you
learned, liked, and any sources of error you may have encountered during the dissection.